1. Field of the Invention
This invention relates to a differential gear unit, usable in the driveline, e.g. a driving axle, of a motor vehicle, which includes a differential carrier, two output gears and a coupling device, preferably a viscous coupling, for restricting differential rotation of the output gears in dependence on the difference in speed therebetween, the coupling device including first and second parts torque-transmittingly connected respectively to one of the output gears or to the differential carrier and to the other output gear of the differential, one of said torque-transmitting connections being established by way of a releasable connecting device.
2. Description of the Prior Art
A differential gear unit of the type above set forth is disclosed in DE-3707106.8. The use of a viscous coupling to inhibit differential rotation between the output elements of the differential gear, the restriction in the differential action being dependent on the difference in speed between the wheels provides improved traction if there is any tendency for a wheel to slip. During braking, however, particularly if there is an anti-locking braking system, the restriction of differential rotation of the wheels is not required because it hinders the action of the anti-locking system. It is therefore known practice to provide an arrangement which cancels the effect of the viscous coupling during braking. In DE-3707106.8 this is achieved by an externally controlled releasable connecting device which disconnects the viscous coupling from one of the output elements of the differential gear unit, when the brakes of the motor vehicle are applied.
When a differential gear unit is incorporated in the drive line between driven front and wheel axles of a vehicle, also disconnection of the viscous coupling is required during braking. Large forces may be required to be applied to actuate the releasable coupling in this case.